In a packet-switched network, data is transmitted in the form of packets processed by routers of the network until they reach their destination. Together the packets for transmission constitute a data stream.
One example of a technology used for routing data packets in packet-switched networks is the multi-protocol label switching (MPLS) technology. The MPLS technology adds one or more labels to the data packet headers, said labels containing information enabling the routers of the network to determine the next hop for a packet to reach its destination. The MPLS technology is described in more detail in Internet Engineering Task Force (IETF) Request For Comments (RFC) 3031.
However, the MPLS protocol can process only packets conforming to the Internet Protocol (IP).
To alleviate this problem, the IETF Pseudo-Wire Emulation Edge-To-Edge (PWE3) standardization group has defined a pseudo-wire concept for emulating a point-to-point link between two equipments of a packet-switched network based on the IP/MPLS technology. Such pseudo-wires, defined in the document RFC 3985, enable data packets to be transmitted that do not conform to the Internet Protocol, for example data packets conforming to the ATM protocol.
Referring to FIG. 1, a pseudo-wire pw1 is set up between a first router T-PE1 at the edge of a packet-switched network PSN and a second router T-PE2, also at the edge of the network PSN. A first link L1 that is part of the pseudo-wire pw1 is set up between the first router T-PE1 and an intermediate router S-PE of the network PSN. A second link L2 that is also part of the pseudo-wire pw1 is set up between the intermediate router S-PE and the second router T-PE2. The first router T-PE1 is called the input router and constitutes a first end of the pseudo-wire pw1 and the second router T-PE2 is called the output router and constitutes a second end of the pseudo-wire pw1. Once the pseudo-wire pw1 has been set up, the input router T-PE1 transmits a data stream routed via the pseudo-wire pw1 to the output router T-PE2. This kind of pseudo-wire pw1 is defined in more detail in the document “An Architecture for Multi-Segment Pseudo-Wire Emulation Edge-to-Edge” (draft-ietf-pwe3-ms-pw-arch-02.txt).
To ensure continuity of service in the event of a fault affecting the output router, the working group PWE3 proposes a solution that backs up the first pseudo-wire set up between the input router and the output router by a second pseudo-wire serving as a back-up pseudo-wire so that, in the event of a fault affecting the output router constituting one end of the first pseudo-wire, the data packets are routed by the back-up pseudo-wire, one end of which consists of a different output router.
Accordingly, referring to FIG. 2, a first pseudo-wire pw1 is set up between an input router T-PE1 at the edge of a packet-switched network PSN and an output router T-PE2 also at the edge of the network PSN. A first link L1 that is part of the pseudo-wire pw1 is set up between the input router T-PE1 and an intermediate router S-PE of the network PSN. A second link L2 that is also part of the pseudo-wire pw1 is set up between the intermediate router S-PE and the output router T-PE2. A second pseudo-wire pw2 is then set up between the input router T-PE1 constituting a first end of the pseudo-wire and a second output router T-PE3 constituting a second end of the pseudo-wire. A first link L3 that is part of the second pseudo-wire is set up between the input router T-PE1 and the intermediate router S-PE and a second link L4 that is also part of the second pseudo-wire is set up between the intermediate router S-PE and the second output router T-PE3.
To ensure continuity of service, the input router T-PE1 includes a function for detecting a fault affecting the first output router T-PE2 and a function for switching the data stream from the first pseudo-wire pw1 to the second pseudo-wire pw2.
A fault affecting the first output router T-PE2 is detected in the input router T-PE1, for example from a message indicating the occurrence of a fault sent via the public switched network PSN to the input router T-PE1. Once informed of the fault, the input router T-PE1 triggers switching of the data stream from the first pseudo-wire pw1 to the second pseudo-wire pw2, thus routing the data to the output router T-PE3.
However, this kind of solution has the drawback of consuming network resources, for example processing resources in the network equipments (storage capacity, computation capacity, etc.), signaling streams for setting-up two pseudo-wires, and bandwidth, especially between the input router and the intermediate router. This increases the restore time in the event of a fault affecting an output router, with a negative impact on quality of service.